Optical passband filter

Abstract: An optical passband filter having a frequency transition that is several of orders or magnitude narrower than prior optical filters includes two frequency routing devices of the arrayed waveguide type. The first frequency routing device has one input for receiving the input signal and at least N outputs, where N corresponds to the number of frequencies that compose the input signal. The N outputs of the first frequency routing device are each coupled to inputs of a second frequency routing device. The outputs of the second frequency device each correspond to one of the selected output frequency bands into which the input signal is to be divided. A multiplexed input signal containing several different frequency channels is divided into bands that are each directed to respective outputs of the second frequency routing device. The frequency transition between the various bands may be as small s the frequency resolution between adjacent outputs of the first frequency routing device.

Description: TECHNICAL FIELD This invention relates to optical communications systems. More particularly, this invention relates to optical passband filters used in optical communications systems. BACKGROUND The capacity and speed of communications systems may be increased by transmitting information in optical form over networks composed of optically transmissive nodes, fibers, and the like. High capacity optical communications systems require that many optical signals be frequency division multiplexed in the components of an optical network. Receivers must be configured such that a desired optical frequency or band of frequencies may be retrieved from a group of many optical frequencies. This necessitates the provision of suitable optical filters such as an optical passband filter for dividing a multiplexed signal composed of a large band of frequencies into one or more sub-bands of frequencies. Until now, there has been no convenient approach to dividing a band of optical frequencies while providing a sharp transition between the pass band and the stop band. The performance of prior optical passband filters has been limited because of the relatively large frequency transition between the pass and stop bands. US-A-4 483 582 discloses a multiwavelength multiplexer-demultiplexer constructed from units each capable of multiplexing or demultiplexing two wavelengths. As many such units as required can be interconnected to form multiplexer-demultiplexers of any number of wavelengths. Each two-wavelength multiplexer-demultiplexer unit includes a four-port power divider, a four-port phase shifter having two ports connected to two ports of the power divider, and another four-port power divider having two ports connected to two ports of the phase shifter. The power divider may be a single-mode optical fiber directional coupler, or a half-silvered mirror beam splitter. The phase shifter may include a pair of single mode optical fibers or a pair of oppositely disposed mirrors. US-A-4 723 829 discloses a device for separating radiation beams of different wavelengths, which emerge from an optical fiber. The device includes a reflection grating and a wavelength-selective mirror. The wavelength-selective mirror is arranged between the optical fiber and the reflection grating. The mirror reflects radiation beams of wavelengths which are spaced comparatively far from the wavelengths of the radiation beams of comparatively closely spaced wavelengths. The reflected beam is reflected to an output fiber. The mirror transmits the radiation beams of comparatively closely spaced wavelengths to the reflection grating. Summary of the Invention A filter according to the invention is as set out in claim 1, preferred forms being set out in the dependent claims. In accordance with this invention, an optical passband filter has been developed. The optical filter in accordance with this invention may be based upon photonic integrated circuitry which can provide very sharp transition between the various passbands which can be realized at a cost lower than that associated with implementing prior optical filters. In one example of the invention, Applicants have realized that integrated optical multiplexers and demultiplexers disclosed, for example, in U.S. Patents 5,002,350 and 5,136,671 may be used to create a monolithic optical passband filter having a frequency transition that is several of orders of magnitude narrower than prior optical filters. In specific terms, this example of the invention comprises two frequency routing devices. The first frequency routing device has one input for receiving the input signal and at least N outputs, where N corresponds to the number of frequencies that compose the input signal. The N outputs of the first frequency routing device are each coupled to inputs of a second frequency routing device. The outputs of the second frequency device each correspond to one of the selected output frequency bands into which the input signal is to be divided. A multiplexed input signal containing several different frequency channels is divided into bands that are each directed to respective outputs of the second frequency routing device. The frequency transition between the various bands may be as small as the frequency resolution between adjacent outputs of the first frequency routing device. If the frequency routing devices disclosed in the above-mentioned patents are employed, this transition may be as sharp as 30dB over 50GHz. This is only an example of the invention. The full scope of the invention entitled to an exclusionary right is set forth in the claims at the end of this application. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram of an example of an optical passband filter in accordance with this invention. FIGS. 2(a) and 2(b) are diagrams of the frequency routing devices employed in the optical passband filter of this invention. FIG. 3 is a diagram of a frequency routing device such as shown in FIG. 2 in which a signal composed of frequencies F 1 and F 2 are input to a first input waveguide and a signal composed of frequencies F 3 and F 4 are input to a second input waveguide. FIG. 4 is a diagram of the frequency routing device shown in FIG. 3 in which the input and output waveguides are reversed. FIG. 5 is a diagram illustrating the details of the frequency routing devices shown in FIG. 1. FIG. 6 is a diagram of an alternative example of an optical passband filter in accordance with this invention. DETAILED DESCRIPTION FIG. 1 shows an example of the optical passband filter of the present invention. It is composed of two frequency routing devices 30 and 32 coupled by waveguides. These structures may be monolithically integrated on a semiconductive wafer. They may be created by known photolithographic techniques. The frequency routing devices employed in the present invention operate as multiplexers and demultiplexers of optical frequencies. The details of their construction and operation are more fully described in U.S. Patent No. 5,136,671 to Dragone. Before the optical passband filter of the present invention is described, the pertinent features of the frequency routing devices will be reviewed. FIGS. 2(a) and (b) show a functional illustration of a routing device 12 of the type employed in the present invention. In general, the routing device may have N input waveguides and M output waveguides to form an NxM frequency routing device. For example, the routing device 12 shown in Fig. 1(a) has four input waveguides 161, 162, 163, 164, and four output waveguides 181, 182, 183, 194. The routing device 12 will demultiplex input optical frequencies directed to any of its input waveguides. For example, as seen in FIG. 2(a), if a multiplexed signal composed of frequencies F 1 , F 2 , F 3 , F 4 , is directed to the first input waveguide 16 1 of the routing device 12, frequency F 1 will be directed to output waveguide 18 1 , frequency F 2 will be directed to output waveguide 18 2 , frequency F 3 will be directed to output waveguide 18 3 , and frequency F 4 will be directed to output waveguide 18 4 . FIG. 2(b) illustrates the manner in which an optical signal is demultiplexed if it is directed to the second input waveguide 16 2 rather than the first input waveguide 16 1 . In this case frequency F 1 will be directed to output waveguide 18 2 , frequency F 2 will be directed to output waveguide 18 3 , frequency F 3 will be directed to output waveguide 18 4 , and frequency F 4 will be directed back up to output waveguide 18, since the routing device 12 only has four output waveguides. If the routing device 12 is provided with a fifth output waveguide 18 5 (shown in dashed lines in FIG. 2(b) to form a 4x5 routing device, frequency F 4 will appear on this fifth output waveguide instead of on the first output waveguide instead of on the first output waveguide 18 1 . FIG. 3 shows a 2x5 routing device in which a signal composed of frequencies F 1 and F 2 is directed to input waveguide 16 1 , and a signal composed of frequencies F 3 and F 4 is directed to input waveguide 16 2 . Based on the properties of the routing device 12 described in connection with FIGS. 2(a) and 2(b), the routing device of FIG. 3 will demultiplex the signals directed to input waveguides 16 1 and 16 2 as shown. Specifically, as expected from FIG. 2(a), frequencies F 1 and F 2 will be directed to output waveguides 18 1 and 18 2 , respectively. Similarly, as expected from FIG. 2(b), frequencies F 3 and F 4 will be directed to output waveguides 18 4 and 18 5 , respectively. It should be noted that no input frequency is directed to output waveguide 18 3 . The routing devices discussed above are symmetrical in operation. Therefore, if in connection with the routing device shown in FIG. 3 frequencies F 1 , F 2 , F 3 , F 4 are directed to waveguides 18 1 , 18 2 , 18 4 and 18 5 , respectively (which now function as input waveguides), frequencies F 1 and F 2 will be output on waveguide 16 1 and frequencies F 3 and F 4 will be output on waveguide 16 2 . Returning to FIG. 1, the optical passband filter 34 of the present invention is composed of two frequency routing devices 30 and 32. The first routing device 30 in which the input signal to be frequency divided is directed is a 1xN routing device, where N is the number of frequencies contained in the multiplexed input signal. The second routing device 32 in which the divided frequency bands are output is an (N+1)x2 routing device. The first set of M output waveguides of the first routing device 30 are each coupled in consecutive order to the first M input waveguides of the second routing device 32. The remaining set of (N-M) output waveguides of the first routing device 32 are each coupled in consecutive order to the remaining input waveguides of the second routing device beginning with the (M+2) input waveguide (i.e., the (M+1) output waveguide of the first routing device is coupled to the (M+2) output waveguide of the second output device). In other words, the (M+1) input waveguide of the second output device is not coupled to the first routing device 30, thus shifting the second set of output waveguides from the first routing device 30 by one relative to the input waveguides of the second input device 32. The manner in which the passband filter 34 operates is most easily understood by recognizing that the second routing device 32 is configured similar to the routing device depicted in FIG. 3 when operating in reverse mode (i.e., when waveguides 18 1 -18 5 in FIG. 3 are employed as input waveguides and waveguides 16 1 and 16 2 are employed as output waveguides). For the sake of clarity, FIG. 4 shows the routing device of FIG. 3 operating in its reverse mode. Returning to FIG. 1, the first routing device 30 demultiplexes the input frequencies F 1 ...F N such that the individual frequencies are consecutively directed to its N output waveguides. The demultiplexed frequencies F 1 ...F N are directed to the inputs of the second routing device 32 in the order described above and shown in FIG. 1. The second routing device 32 then multiplexes the frequencies F 1 ...F N just as depicted in FIG. 4; the first FM frequencies appear on the first output line 36 1 and the second F( M+1 )...F N frequencies appear on the second output line 36 2 . Accordingly, the net result produced by the two routing devices 30 and 32, which form the optical passband filter 34, is that the multiplexed signal composed of frequencies F 1 ...F N is divided into two signals, the first composed of frequencies F 1 ...F M and the second composed of frequencies F( M+1 )...F N FIG. 5 shows the pertinent details of each routing device 30 and 32 shown in FIG. 1. Both of those devices may have identical configurations except for, possibly, the differing numbers of input and output waveguides that are employed. Each frequency routing device contains a plurality of input waveguides 26 connected to a free space region 28. A plurality of output waveguides 40 extends from the free space region 28 and is connected to an optical grating 42. The optical grating 42 comprises a plurality of unequal length waveguides which provides a predetermined amount of path length difference to a corresponding plurality of input waveguides 44 connected to another free space region 46. The free space region 46 is connected to a plurality of output waveguides 38. These frequency routing devices operate as multiplexers and demultiplexers of optical frequencies. As noted above, details of their construction an operation are more fully described in the U.S. patent No. 5,136,671 to Dragone. In the case of the frequency routing device 30 seen in FIG. 1, the input waveguide 16 corresponds to one of the input waveguides 26 shown in FIG. 5. In the case of the frequency routing device 32 seen in FIG. 2, the output waveguides 36 1 and 36 2 correspond to two of the output waveguides 38 shown in FIG. 5. The above-described embodiment of the present invention can be generalized to divide an input signal into more than two frequency bands. In the generalized case, if W is the number of frequency sub-bands into which the input signal is to be divided, the second routing device 32 must have at least (W-1) additional input waveguides relative to the number of input waveguides of the first routing device 30. FIG. 6 shows an example of an optical band pass filter for dividing a signal into three sub-bands (i.e., W = 3). In this case the first routing device 30 has six output waveguides 18 1 , 18 2 , ...,18 6 , and thus the second routing device 32 must have eight input waveguides. In a preferred embodiment of the invention, the number of output waveguides of the first routing device is equal to the number of input waveguides of the second routing device. In particular, if the passband filter is to divide a signal into W frequency sub-bands and the number of multiplexed frequencies forming the input signal is N (so that the first routing device must have at least N output waveguides), the second routing device must haveN+(W-1) input waveguides. While in this case the first routing device is only required to have N output waveguides, the complexity of the device is significantly reduced if it is instead provided with N+(W-1) output waveguides, where the additional W-1 waveguides are unused. By employing such a configuration, the width Δf of the passband directed to each of the N output waveguides of the first routing device will be equal to the width Δf of the passband that may be directed to each of input waveguides of the second routing device over the entire free spectral range period NΔf. In the example of the invention shown in FIG. 6, the number of output waveguides of the first routing device and the number of input waveguides of the second routing device are both equal to eight, where the final two input waveguides 18 7 and 18 8 of the first routing device are unused.

Claims: An optical passband filter comprising an input frequency routing device coupled to an output frequency routing device, CHARACTERIZED IN THAT : said input frequency routing device (30) is an integrated optical demultiplexer having an input waveguide (16) for receiving a frequency multiplexed signal composed of a plurality of optical frequencies F 1 , ...F N and having N output waveguides, N > 1, each of said N output waveguides outputting one of the plurality of optical frequencies, said N output waveguides formed into a plurality of W output sets, each output set including a contiguous group of said N output waveguides, each output set being associated with a different band of the plurality of optical frequencies; and said output frequency routing device (32) is an integrated optical multiplexer having W output waveguides (36 1 , 36 2 ) and having (i) N input waveguides divided into a plurality W of input sets (1 ... M, [M+2] ... [N + 1]) and (ii) W-1 unconnected input waveguides, each unconnected input waveguide located between the waveguides of a different pair of the W input sets, each input set being formed from a contiguous group of said N input waveguides, the waveguides of each input set being coupled to the waveguides of a different output set of said input frequency routing device, wherein only N output waveguides of the input frequency routing device are coupled to said N input waveguides of the output frequency routing device, the input frequency routing device is arranged so that the individual frequencies F 1 , ...F N are consecutively directed to the N output waveguides of the input frequency routing device, the N output waveguides are coupled to the N input waveguides of the output frequency routing device so that the frequencies F 1 , ...F M appear on a first output waveguide (36 1 ) and the remaining the frequencies F M+1 ,...F N appear on the remaining W-1 output waveguides (36 2 ). The filter of claim 1 further comprising: N waveguides connecting the N outputs from the input frequency routing device to the N inputs of the output frequency routing device, and wherein said output frequency routing device has a total number of inputs equal to N+W-1, where W is the number of selected frequency bands into which the multiplexed optical frequencies are to be divided. The filter of claim 2 wherein each of the N waveguides has a first end coupled to a respective one of the N outputs from the input frequency routing device and a second end coupled to a respective one of the inputs of the output routing device such that said output frequency routing device has W-1 uncoupled inputs uncoupled to any of said N outputs of the input frequency routing device, whereby each of said uncoupled inputs corresponds to a frequency at which said plurality of multiplexed optical frequencies is divided into one of said plurality of selected output frequency bands. The filter of claim 2 or claim 3 wherein the input frequency routing device has N+W-1 outputs, namely, the N outputs recited in claim 1 and W-1 further outputs (18 7 and 18 8 ) uncoupled to any of said N inputs of the output frequency routing device. The filter of any of the preceding claims wherein the number of said input sets is equal to two, the number of said output sets is equal to two and said output frequency routing device has two outputs.

Inventors: GLANCE BERNARD, WILSON ROBERT WOODROW, GLANCE, BERNARD, WILSON, ROBERT WOODROW
Applicants: AT & T CORP, AT&T CORP.
Application year: 1994
Main classification: G02B6
IPC codes: H04B10, H04Q11, H04J14, G02B6